The inventors have observed that conventionally used materials used to form work function layers in p-type metal oxide semiconductor (pMOS) transistors provide a work function that is inadequate and limits the speed of operation of the pMOS transistor. For example, conventionally used work function metals utilized to form work function layers of pMOS transistors typically provide a band gap value of about 4.6 eV. However, the inventors have observed that a higher work function (e.g., a band-edge work function of at least 4.9 eV) is necessary for the formation of a p-type metal oxide semiconductor (pMOS) transistor to provide sufficient speed for current semiconductor technologies.
Accordingly, the inventors have provided improved methods for forming metal gate structures.